Fuel spitting out of air filters at full power

by Neil Laubach in 912 / 914 Technical Questions 12 years ago

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Re: Fuel spitting out of air filters at full power

by zeb carris » 8 years ago

zeb carris

2 15 3

Hi Roger,
thanks for your considerations.
In setting 5200 WOT I thought to correctly apply the indications of the SL-912-016 par. 3.1.2 Performance recommendations

Please consider that usually I fly at 5000-8000 ft. At that altitude setting 5200 according your procedure (reducing the MAP from WOT) would reduce my flyght cruise speed as I am reducing the already altitude reduced power (not having the turbo charger).

About the Carburetors I did all the checks you indicated in the old carburetors (including the epoxy on the ports) but without positive result. Then I decided to buy brand new carburators. They are perfectly tuned both for the float level and sync (pneumatic and mechanic).
I have been using these new carbs for about two hours now (sufficient to see that the problem is still there).
No particular vibrations at idle or cruise.
I used the vent tubes inside the air filters (I have been running for 1400 hours in this configuration) and with the new carbs in between the clip and the float chamber.
The reasons for trying 100 octane fuel is that here in Italy there is a suspicion that the regular Mogas may be additivated with products that do not properly burn...
I add some picture of the stains.
Sebastiano

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Re: Fuel spitting out of air filters at full power

by Roger Lee » 8 years ago

Roger Lee

127 5.2k 4.78k

Hi Zeb,

Do the vent tubes have fuel residue in them? That doesn't look like a active leak. The picture looks like a dried fuel stain that may have happened once. Do you notice live fuel in the drip tray and do you notice it only at low rpms like idle or at high rpms or both. Can you actually see fuel drip from the air filter?
I had a fuel leak once that a customer thought was coming out the air filter and it ended up being the fuel inlet fitting and the fuel running back off the bottom of the air filter.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel spitting out of air filters at full power

by Roger Lee » 8 years ago

Roger Lee

127 5.2k 4.78k

The #3 line you have underlined does not say anything about what the overall prop pitch and WOT setting should be. It's just a blanket statement. Only that Rotax would rather you don't run the engine down under 5200 continuously.

This leaves two scenarios.
1. Have the pitch so coarse that the engine can only get 5200 WOT no matter what.
This was done in the US for a few years when the LSA movement started here. The aircraft MFG's were told to quit doing this and some experienced crankcase cracks. Most all now set the WOT rpm to get at least 5500 rpm at WOT and then reduce throttle to your deired cruise rpm.

2. Have the prop pitch set to get 5500+ rpm at WOT and then throttle back to 5200 rpm.

The second one is what is more desirable and the first one avoid due to excessive stress on the engine.

Over pitching a prop has zero benefits and has nothing, but negatives for flight characteristics. I can say this from good solid experience. I did a research project years ago for a couple prop MFG's and have set up hundreds of props.

It is possible that you may even be causing your fuel issue with this prop setup from induced stress vibration. Try setting the prop to get a minimum of 5500 rpm WOT at whatever altitude you are flying at and then throttle back to 5200 rpm. The engine will be less stressed, work less hard, get better fuel economy, less vibration and possibly better speed at reduced throttle settings.

If I set my prop to get 5600-5650 rpm at WOT at the same altitude as you I will have a faster top speed and walk away from you, I will use less fuel, have a cooler engine temps and when you are flying at 5200 WOT at that altitude I will be flying at less rpm and using less fuel to equal your speed.

You aren't getting max performance or good efficiency out of your engine when it is set to only get 5200 RPM WOT. You don't have the HP or torque with this course a prop pitch to turn it efficiently.
There is nothing good to be gained from over pitching a prop.

Case in point:

I flew my Flight Design CT over to another airport to pick up another Flight Design to bring back to my airport for an annual. The CT I was flying could only get 5200 rpm WOT and mine could get 5650 rpm at cruise altitude. We took off and the other CT plane ran away in climb and speed at take off and it left the ground much sooner. I had to radio my buddy to slow down. After I caught up I was running 5150 rpm WOT to keep up with my plane and it was only running at 4650 rpm to get the same speed. My engine temps were all higher and my fuel consumption was 1.5 GPH more. The engine was working way to hard and not running efficiently.
During the annual I set the prop pitch to get the same rpm as mine. After that he took off faster, flew faster, had lower engine temps, climbed better and flew faster. There are no redeeming qualities in flight performance with an over pitched prop regardless of altitude.

Try it before you condemn it.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel spitting out of air filters at full power

by zeb carris » 8 years ago

zeb carris

2 15 3

Hi Roger,
I do not see fuel dripping out from the air filter or the vent tube. After cleaning every surface I can see the stains only if I fly for at least 45 minutes at cruise. I find the bottom of filters little bit wet after that flight.
Maybe some drops collect on the bottom of the filters and then are spreaded all around by the airflow in the engine compartment.
After that flight the stains are not so intense as the one shown in the picture. I agree with you on the fact that the stain shown may have collected and dried in a long time.
The problem has not shown for about 1400 hours flying always in the same manner.
It is about from 100 hours (two years) that I notice that problem, even having changed the carbs with new ones.

I have an electric inflight variable pitch propeller and I can get whatever rpms I want.
I will try the procedure that you indicate and I tell you the results.
Sebastiano

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Re: Fuel spitting out of air filters at full power

by Roger Lee » 8 years ago

Roger Lee

127 5.2k 4.78k

If you installed new carbs someone should have checked the float level before the install. If this wasn't done I would take the time to do it to know it is right and to rule this as the problem in or out. If the carbs have been on for a year I would also remove the floats and weigh them. The set should not weigh over 7 grams.
What makes me suspicious of the float height and float buoyancy is you are getting fuel in the vent tube. Excessive vibration will also cause fuel to vent.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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